This study investigates chlorapatite [ideally Ca10(PO4)6Cl2)] from two spatially separated locations in the Odegardens Verk apatite mine, Bamble Sector, SE Norway, which was partially metasomatized under amphibolite-facies conditions. Relative to the original chlorapatite, metasomatized areas in sample DL136 are enriched in OH and F, depleted in Na, Si, and (Y + REE) and contain numerous inclusions (1–15 μm) of monazite and xenotime. In contrast, metasomatized areas in sample TN174 are enriched in F and Si, depleted in Na, show (Y + REE) abundances similar to the original chlorapatite, and host only a few monazite and xenotime grains.

In order to define the role of fluids in the formation of (Y + REE)-phosphate minerals associated with apatite, a series of experiments covering wide ranges of temperature (900–300 °C) and pressure (1000–500 MPa) for a series of fluid compositions (pure H2O, a 50/50 molar mix of H2O and CaF2, and CO2 with 1–2 wt% H2O) were conducted on unaltered chlorapatite from the Odegardens Verk. In the H2O experiments, features due to metasomatic alteration [depletion in Na, Si, and (Y + REE) as well as numerous monazite and xenotime inclusions], as observed in the natural sample DL136, were fully reproduced. Monazite and xenotime grains were also observed growing on the surface of the apatite in dissolution embayments in metasomatized areas. In the 50/50 CaF2-H2O experiments, metasomatized regions are strongly enriched in F and Si, depleted in Na, show unchanged (Y + REE) abundances, and contain only a very few, small monazite and xenotime grains similar to what was observed in sample TN174.

Natural and experimental observation imply the presence of two coupled substitutions: Na+ + (Y + REE)3+ = 2 Ca2+ and Si4+ + (Y + REE)3+ = P5+ + Ca2+. In the case of DL136 and H2O–fluid experiments, Na+ and Si4+ went into solution, whereas the released (Y + REE) contributed to the growth of monazite and xenotime within the metasomatized apatite structure. For sample TN174 and the 50/50 CaF2 + H2O–fluid experiments, Na in apatite became depleted but Si was enriched thereby stabilizing the (Y + REE) in the now recrystallized fluor-hydroxylapatite structure, which retarded the growth of monazite or xenotime inclusions. The basic conclusion of this study is that nucleation of monazite and xenotime in the metasomatized chlorapatite is principally a function of the composition of the infiltrating fluid and, to a much lesser extent, temperature and pressure.

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Fluid-induced nucleation of (Y + REE)-phosphate minerals within apatite: Nature and experiment. Part I. Chlorapatite

Apatite is a superb mineral by which to investigate the nature of fluids that have passed through and altered a rock (metasomatic processes). Its ubiquity allows it to act as a reservoir for P, F, Cl, OH, CO2, and the rare earth elements. It is also a powerful thermochronometer and can be chemically altered by aqueous brines (NaCl–KCl–CaCl2–H2O), pure H2O, and aqueous fluids containing CO2, HCl, H2SO4, and/or F. Thus, apatite is the perfect tracker of metasomatic fluids, providing information on the timing and duration of metasomatism, the temperature of the fluids, and the composition of the fluids, all of which can feed back into the history of the host rock itself.

Apatite: A Fingerprint for Metasomatic Processes

Partitioning behavior of chlorine and fluorine in the system apatite-silicate melt-fluid

solidification of the interstitial liquid. The stratigraphic distribution Halogen-bearing minerals, especially apatite, are minor but ubiof S, Cu and the PGE in the Critical Zone cannot readily be quitous phases throughout the Bushveld Complex. Interstitial apatite explained either by precipitation of sulfide as a cotectic phase or as is near end-member chlorapatite below the Merensky reef (Lower a function of trapped liquid abundance. Evidence from potholes and and Critical Zones) and has increasingly fluorian compositions with the PGE-rich Driekop pipe of the Bushveld Complex imply that increasing structural height above the reef (Main and Upper Zones). migrating Cl-rich fluids mobilized the base and precious metal Cl/F variations in biotite are more limited owing to crystal-chemical sulfides. We suggest that the distribution of sulfide minerals and controls on halogen substitution, but are also consistent with a the chalcophile elements in the Lower and Critical Zones reflects a decrease in the Cl/F ratio with structural height in the complex. general process of vapor refining and chromatographic separation of A detailed section of the upper Lower Zone to the Critical Zone is these elements during the evolution and migration of a metalliferous, characterized by an upward decrease in sulfide mode from 0·01– Cl-rich fluid phase. 0·1% to trace–0·001%. Cu tends to correlate with other incompatible elements in most samples, whereas the platinum-group elements (PGE) can behave independently, particularly in the Critical Zone. The decrease in the Cl/F ratio of apatite in the

The Halogen Geochemistry of the Bushveld Complex, Republic of South Africa: Implications for Chalcophile Element Distribution in the Lower and Critical Zones

Mineral compositional constraints on petrogenesis and oxide ore genesis of the late Permian Panzhihua layered gabbroic intrusion, SW China

Apatite from the Great Dyke of Zimbabwe is relatively rich in the hydroxy-fluorapatite end-members. The mole fraction of fluorapatite increases from approximately 40% in cumulates of the Ultramafic Sequence to over 60% in a sample near the top of the exposed Mafic Sequence. The chlorapatite component decreases from a typical high of 10–20 mole% in the Ultramafic Sequence to about 1% in the uppermost part of the Mafic sequence. However, within-sample variation may be as great as the entire stratigraphic variation. Halogen contents of marginal samples generally are similar to axial samples, but tend not to have as high Cl concentration and tend to OH-enrichment. Biotite compositions approach hydroxyl end-member compositions, and apatite-biotite OH-F exchange geothermometers give an average closure temperature of 564° C. Apatite from the Umvimeela Dyke, an unlayered dike that parallels the Great Dyke over much of its length, contains less Cl than is seen in the Ultramafic Sequence cumulates of the Great Dyke. While the overall stratigraphic trend is characterized by a decrease in the Cl/F ratio with stratigraphic height, within the P1 unit at the top of the Ultramafic Sequence there is a positive correlation between Cl and other incompatible elements such as Na and Ce. The apparent contradiction between the general stratigraphic trend of decreasing Cl/F ratio with fractionation and the apparent increase in Cl and other incompatible elements seen in the P1 unit can be explained by assuming that the Great Dyke magma chamber was degassing near its top, where confining pressure was lowest and Cl was preferentially lost to a separating volatile-rich fluid. As cumulates formed on the floor, they entrapped liquid that was increasingly depleted in Cl at the higher stratigraphic levels. However, at any given stratigraphic interval, either local fluid enrichment or the eventual crystallization of halogen-bearing minerals that incorporate the smaller F ion in preference to the larger Cl ion led to a local increase in the Cl/F ratio.

C. Love

Papers

Halogen geochemistry of the Great Dyke, Zimbabwe